17.2: clarify language (hadley#1637)

"POSIXct" is the thing tripping off the tongue in this sentence, not base R.

datetimes.qmd

@@ -55,7 +55,7 @@ There are three types of date/time data that refer to an instant in time:
 
 -   A **date-time** is a date plus a time: it uniquely identifies an instant in time (typically to the nearest second).
     Tibbles print this as `<dttm>`.
-    Base R calls these POSIXct, but doesn't exactly trip off the tongue.
+    Base R calls these POSIXct, but that doesn't exactly trip off the tongue.
 
 In this chapter we are going to focus on dates and date-times as R doesn't have a native class for storing times.
 If you need one, you can use the **hms** package.